This invention relates to a light-receiving device which converts the received light signal to a voltage signal.
In recent years, a great demand has existed for light-receiving devices due to the development of optical communications. FIG. 1 shows the circuit arrangement of the prior art light-receiving devices. This devices comprises a photo-detector 10, preamplifier 12, variable attenuating circuit 14, and main amplifier 16. Coupling capacitors 18, 20, 22 are connected between the respective series-connected elements 10, 12; 12, 14; 14, 16. The photo-detector 10 comprises a photodiode 24 and a resistor 26 connected in series between a power source V and a ground. The photodiode 24 is set near one end of an optical fiber 30 acting as a light transmitting line. A light signal entering the photodiode 24 is converted to a current signal by the photo-detector 10. The current signal from the photo-detector 10 is supplied to the preamplifier 12 through the coupling capacitor 18. The preamplifier 12 comprises the series connection of transistors 32, 34. This preamplifier 12 amplifies a base current signal input to the base and sends forth an amplified voltage signal. The voltage signal from the preamplifier 12 is supplied to a variable attenuating circuit 14 through the coupling capacitor 20. The variable attenuating circuit 14 is a T-type attenuating circuit formed of PIN diodes 36, 38, 40. The PIN diodes 36, 38 are connected in series. A capacitor 42 is connected between the node of the PIN diodes 36, 38 and the cathode of the PIN diode 40. The variable attenuating circuit 14 varies the ratio that an input voltage is divided by changing the resistance components of the PIN diodes 36, 38, 40, thereby changing the attenuation level of an input voltage. Constant current sources 44, 46, 48 for controlling the variable attenuating circuit 14 are respectively connected to the PIN diodes 36, 38, 40. An output voltage signal from the variable attenuating circuit 14 is supplied to the main amplifier 16 through the coupling capacitor 22. The main amplifier 16, which includes a transistor 50, sends forth an input voltage in an amplified state. An output voltage signal from the main amplifier 16 is supplied to an external device as a voltage signal corresponding to the light entering the photodiode 24.
FIG. 2A shows an equivalent circuit of the variable attenuating circuit 14 of FIG. 1. Reference character V1 shows a power source for the preamplifier 12, and reference character R1 denotes an output impedance of the preamplifier 12. The output impedance R1 of the preamplifier 12 which sends forth a voltage output substantially indicates 0 .OMEGA.. Variable resistances R2, R3, R4 respectively indicate the resistance components of the PIN diodes 36, 38, 40. Reference character R5 represents an input impedance of the main amplifier 16. Reference characters C1, C2, C3 denote floating capacitances.
It is now assumed that R2=R3 and R2 (=R3)&lt;&lt;R4. The equivalent circuit of FIG. 2A is changed to that of FIG. 2B. If the resistances R2, R3 are both 0 .OMEGA., the time constant .tau. is measured as follows, since the resistance R1 is much lower than the resistances R4, R5. EQU .tau.=R1(C1+C2+C3) (1)
Since the resistance R1 substantially indicates 0 .OMEGA., the frequency response is only slightly deteriorated by the floating capacitances C1, C2, C3.
Assuming, however, R2 (=R3)&gt;&gt;R4, an equivalent circuit represents the circuit of FIG. 2C. If the resistance R1 and floating capacitance C1 are disregarded, the time constant .tau. is determined as follows: ##EQU1## The term ##EQU2## denotes a resistance of hundreds of .OMEGA.s to kilos of .OMEGA.s. Therefore, the floating capacitance C3 substantially affects the frequency response. If ##EQU3## a cutoff frequency ##EQU4##
As described above, the conventional lightreceiving device has the following drawbacks: (1) the floating capacitance reduces the frequency response, thereby resulting in the deterioration of the waveform of a high frequency signal, causing unreliability in the light-receiving device; and (2) since an output current signal from a photo-detector is converted to a voltage signal by a preamplifier, and the voltage signal is further amplified by the main amplifier, a circuitry involving the preamplifier and other elements is considerably complicated.